Femtosecond Soft X-ray Absorption Spectroscopy Identifies Metal-Centered S₁ Excited State of Cyanocobalamin

Time-resolved X-ray absorption spectroscopy (TRXAS) at the Co L₃-edge was used to identify metal-centered (MC) character in the S₁ excited state of cyanocobalamin (CNCbl). Cobalamins have UV/visible spectra that are dominated by intense corrin-based excitations, but these ligand-centered states energetically overlap with charge transfer and MC excited states that may be populated following photoexcitation. Ultrafast optical and hard X-ray spectroscopy have shown that CNCbl forms a structurally distorted S₁ state, but these probes lack a clear signature of the S₁ electronic identity, which theory has suggested is a ligand-to-metal charge transfer (LMCT) state. Femtosecond soft X-ray TRXAS offers greater state-selectivity than many optical or hard X-ray probes but has, so far, been limited to highly concentrated (≥100 mM) samples. A new experimental setup at the European X-ray Free Electron Laser (EuXFEL) that enables studies of sub-10 mM samples and provides ∼100 fs time-resolution is used to measure the TRXAS of CNCbl at the Co L₃-edge. Comparison of the L₃-edge XAS spectrum measured at 0.8 ps with ligand field multiplet simulations indicates that the S₁ state is primarily an MC excited state. The sub-20 μOD detection sensitivity achieved in this study demonstrates the possibility of applying this method to a wide range of naturally occurring and synthetic transition metal complexes.